Learning Objective
- Describe how properties of solutions influence the movement of water through membranes.
Osmosis
Osmosis describes a net movement of water molecules through a semi-permeable membrane from the side where water concentration is higher/solute concentration is lower to the side where water concentration is lower/solute concentration is higher. This can be viewed as an attempt to equalize the concentrations of all substances on both sides of the membrane, maximizing entropy (disorder) and reaching a lower energy state. A semi-permeable membrane is a barrier that smaller molecules can pass through but larger molecules and colloids cannot. Colloids are very large molecules such as proteins and starches that remain suspended indefinitely in water but are not truly dissolved because of their size. Since large molecules and colloids cannot pass through the membrane, it is impossible to equalize their concentrations on opposite sides of the membrane, resulting in osmotic pressure. Osmotic pressure of a solution is the amount of pressure that would have to be applied to a solution prevent pure water from passing through a semi-permeable membrane into the solution. Figure 9.6 shows a practical way to measure osmotic pressure employing a u-shaped tube with two chambers separated a semi-permeable membrane. The chambers are filled to equal heights, one side with pure water and the other with the solution of interest. As water molecules cross the membrane, the liquid level rises on the right. As the height differential increases, the column of liquid on the right pushes back against the membrane. When the push of water molecules on the membrane due to water’s higher concentration on the left is exactly balanced by the push of gravity from the higher column of liquid on the right, equilibrium is reached. Thus the height of the column of liquid on the right indicates the osmotic pressure of the solution.
Reverse Osmosis
Counterpressure exerted on the side with more solute molecules will reduce or halt the net movement of water through the semi-permeable membrane. An even higher pressure can be exerted to force solvent from side with more solute to the side with less solute, a process called reverse osmosis. Reverse osmosis is used to make potable water from saltwater where sources of fresh water are scarce. Reverse osmosis can also play a role in producing real maple syrup which contains over 65% sugar from maple sap which contains only about 2% sugar. Reverse osmosis is used to remove some of the water from the sap, followed by boiling to drive off more of the remaining water to concentrate the the sugar and other flavor components.
Dialysis
In dialysis, an aqueous mixture is placed within a tube or bag made of a semi-permeable membrane, and the tube or bag is in tun placed in pure water. Small molecules and ions are able to move through tiny pores in the membrane, but the molecules of colloidal materials such as proteins and starch are too large to leave the tube or bag. This is one method used to purify proteins by removing many small molecules and ions that may be found with the proteins in living systems.
To Your Health: Dialysis
The main function of the kidneys is to filter the blood to remove wastes and extra water, which are then expelled from the body as urine. Some diseases rob the kidneys of their ability to perform this function, causing a buildup of waste materials in the bloodstream. If a kidney transplant is not available or desirable, a procedure called dialysis can be used to remove waste materials and excess water from the blood.
In one form of dialysis, called hemodialysis, a patient’s blood is passed though a length of tubing that travels through an artificial kidney machine (also called a dialysis machine). A section of tubing composed of a semipermeable membrane is immersed in a solution of sterile water, glucose, amino acids, and certain electrolytes. The osmotic pressure of the blood forces waste molecules and excess water through the membrane into the sterile solution. Red and white blood cells are too large to pass through the membrane, so they remain in the blood. After being cleansed in this way, the blood is returned to the body.
Dialysis is a continuous process, as the osmosis of waste materials and excess water takes time. Typically, 5–10 lb of waste-containing fluid is removed in each dialysis session, which can last 2–8 hours and must be performed several times a week. Although some patients have been on dialysis for 30 or more years, dialysis is always a temporary solution because waste materials are constantly building up in the bloodstream. A more permanent solution is a kidney transplant.
Cell membranes are semipermeable, so the osmotic pressures of the body’s fluids have important biological consequences. If solutions of different osmolarity exist on either side of the cells, solvent (water) may pass into or out of the cells, sometimes with disastrous results. Consider what happens if red blood cells are placed in a hypotonic solution, meaning a solution of lower osmolarity (less solutes, more water) than the liquid inside the cells. The cells swell up as water enters them, disrupting cellular activity and eventually causing the cells to burst. This process is called hemolysis. If red blood cells are placed in a hypertonic solution, meaning one having a higher osmolarity (more solutes, less water) than exists inside the cells, water leaves the cells to dilute the external solution, and the red blood cells shrivel and die. This process is called crenation. Only if red blood cells are placed in isotonic solutions that have the same osmolarity as exists inside the cells are they unaffected by negative effects of osmotic pressure. Glucose solutions of about 0.31 M, or sodium chloride solutions of about 0.16 M, are isotonic with blood plasma.
Osmotic pressure explains why you should not drink seawater if you are abandoned in a life raft in the middle of the ocean. Its osmolarity is about three times higher than most bodily fluids. You would actually become thirstier as water from your cells was drawn out to dilute the salty ocean water you ingested. Our bodies do a better job coping with hypotonic solutions than with hypertonic ones. The excess water is collected by our kidneys and excreted.
Osmotic pressure effects are used in the food industry to make pickles from cucumbers and other vegetables and in brining meat to make corned beef. It is also a factor in the mechanism of getting water from the roots to the tops of trees!
Career Focus: Perfusionist
A perfusionist is a medical technician trained to assist during any medical procedure in which a patient’s circulatory or breathing functions require support. The use of perfusionists has grown rapidly since the advent of open-heart surgery in 1953.
Most perfusionists work in operating rooms, where their main responsibility is to operate heart-lung machines. During many heart surgeries, the heart itself must be stopped. In these situations, a heart-lung machine keeps the patient alive by aerating the blood with oxygen and removing carbon dioxide. The perfusionist monitors both the machine and the status of the blood, notifying the surgeon and the anesthetist of any concerns and taking corrective action if the status of the blood becomes abnormal.
Despite the narrow parameters of their specialty, perfusionists must be highly trained. Certified perfusion education programs require a student to learn anatomy, physiology, pathology, chemistry, pharmacology, math, and physics. A college degree is usually required. Some perfusionists work with other external artificial organs, such as hemodialysis machines and artificial livers.
Concept Review Exercises
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What is osmosis?
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What is meant by “semi-permeable membrane”?
Answers
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Osmosis is the net movement of water molecules across a semi-permeable membrane from an area of low solute concentration to an area of high solute concentration.
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A semipermeable membrane is a thin structure with microscopic pores that allow only some substances to pass through.
Key Takeaway
- Movement of water through a semi-permeable membrane is based on concentration differences (gradients) and can have profound impacts on cells.
Exercises
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Why can’t colloidal substances pass through a semi-permeable membrane?
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Why does glucose (C6H12O6) pass through a certain semi-permeable membrane but raffinose (C18H32O16) does not?
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See Figure 9.8. If the membrane bag contains NaCl, glucose, and protein at the beginning, which substance(s) would be found in the water in the beaker at equilibrium? Explain.
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See Figure 9.8. If the membrane bag contains LiF, glycerol (C3H8O3) , and starch at the beginning, which substance(s) would be found in the water in the beaker at equilibrium? Explain.
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What would happen to red blood cells placed in a 0.50 M glucose solution? Hint: see sentence just before Figure 9.9.
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What would happen to red blood cells placed in a 0.10 M NaCl solution? Hint: see sentence just before Figure 9.9.
Answers
1. The molecules of a colloidal substance are too large to truly dissolve or to pass through a membrane. They are also large enough to scatter a beam of light!
3. Salt and glucose would be found in the water in the beaker at equilibrium. Each is small enough to pass through the microscopic pores in the semi-permeable membrane, following their concentration gradients. Protein also has a higher concentration inside the bag at the beginning, but it is colloidal, its molecules are too large to pass through the membrane’s pores.
5. a. 100.5°C
b. 102.3°C
c. 101°C
7. 0.31 M glucose solution is isotonic with blood, so a 0.50 M glucose solution is hypertonic with blood. There will be a net movement of water out of the red blood cells, resulting in crenation or shriveling of the cells.
